The present invention relates in general to a camshaft for an engine, and more particularly to a structure of such an engine camshaft using a ceramic material, which is improved in wear resistance and durability and is reduced in weight.
In the valve train of a car engine, wear and abrasion of the components, particularly of cams and rocker arms having sliding surfaces, are a serious problem that should be overcome for improving the durability of the engine. These cams used in the valve train of an engine are conventionally made of cast iron, and their sliding surfaces are chilled or hardened by heat treatment, to increase the wear resistance thereof. Efforts have been made to enhance the durability and reliability of the rocker arms. For example, the sliding surfaces of the rocker arms made of cast iron can be nitrided or coated with a hard-chrome plating layer. Further, sintered alloy tips having an excellent wear resistance are used for the rocker arms which are made of aluminum.
Keeping pace with recent technological progress, engines of automotive vehicles are increasingly required to provide higher performance and capability. On the other hand, there has been a continuing need for controlling emissions from the engine. As a result of meeting the emission control requirements, the lubricating conditions or environments of the engine are aggrevated. Under these circumstances, further improvements are demanded of the cams which are formed as longitudinally spaced-apart integral sections of a camshaft, and of the rocker arms in sliding contact with the camshaft, so as to increase the wear resistance of their sliding surfaces, more specifically, resistance to pitting and scuffing of the sliding surfaces.
To this end, various studies have been made on materials for use as camshafts and rocker arms. Above other things, particular attention of the industry has been paid to a composite camshaft, the cam portions of which are made of a highly wear-resistant material that is different from the material of the shaft portion or portions. In general, it is considered that the cam portions of such a composite camshaft may be made of a ceramic material. However, a ceramic material for the cam portions and a metallic material for the shaft portion or portions have very different properties. Therefore, there arises a problem of how these two different materials are joined or united into an integral assembled composite camshaft.
In the case where metallic cam portions are formed integrally on a metallic shaft portion, the following methods are commonly practiced: Cams of sintered alloy, for example, are set in a casting mold for a camshaft, and a suitable molten metal for the shaft portion is poured into the mold, whereby a composite camshaft with the cam and shaft portions of different metals is obtained. Alternatively, the cams of sintered alloy are formed with bores, and a suitable pipe or shaft of steel or other metal is passed through the bores of the cams, so that the cams are secured to the pipe or shaft by way of a press fit, diffusion joint, welding, brazing, etc. In the case of fabricating a composite camshaft with ceramic cam portions, however, the casing method is difficult to practice because of cracking due to casting stress. Further, the press fit process of forcing a steel pipe or shaft into the bores of the ceramic cams is not desired, since the ceramic cams are subject to tensile stresses, to which ceramic materials inherently exhibit a relatively low resistance. At present, other methods such as diffusion joint, welding and brazing are not practically applicable to ceramic materials. In this connection, it is recognized that the need for a finishing process on the ceramic cam portions of a composite camshaft should be kept to a minimum, because of high hardness nature of a ceramic material, which makes the finishing process difficult.
In manufacturing a camshaft with ceramic cam portions, it is essential to prepare ceramic cam parts and metallic shaft parts with high dimensional accuracy, and join or unite these ceramic and metallic parts with high precision, in a reliable manner.